Polymer membranes have been proposed for various separations. It has been found that different molecules can be made to permeate through selected polymers differently. For example if one component of a mixture is found to permeate though a polymer rapidly and a second component is found to permeate through the polymer more slowly or not at all, the polymer may be utilized to separate the two components. Polymer membranes potentially can be used for gas separations as well as liquid separations.
Numerous research articles and patents describe polymeric membrane materials (e.g., polyimides, polysulfones, polycarbonates, polyethers, polyamides, polyarylates, polypyrrolones, etc.) with desirable gas separation properties, particularly for use in oxygen/nitrogen separation (See, for example, Koros et al., J. Membrane Sci., 83, 1-80 (1993); and Du et al., Macromol. Rapid Commun., 32, 631-636 (2011), hereinafter referred to as the Du Article.
The polymeric membrane materials are typically used in processes in which a feed gas mixture contacts the upstream side of the membrane. This results in a permeate mixture on the downstream side of the membrane with a greater mole fraction of one of the components than the composition of the original feed gas mixture. A pressure differential is maintained between the upstream and downstream sides, providing the driving force for permeation. The downstream side can be maintained as a vacuum, or at any pressure below the upstream pressure.
Membrane performance may be characterized by the flux of a gas component across the membrane. This flux can be expressed as a quantity called the permeability (P), which is a pressure- and thickness-normalized flux of a given component. The separation of a gas mixture is achieved by a membrane material that permits a faster permeation rate for one component (i.e., higher permeability) over that of another component. The efficiency of the membrane in enriching a component over another component in the permeate stream can be expressed as a quantity called selectivity. Selectivity can be defined as the ratio of the permeabilities of the gas components across the membrane (i.e., PA/PB, where A and B are the two components). A membrane's permeability and selectivity are material properties of the membrane material itself, and thus these properties are ideally constant with feed pressure, flow rate and other process conditions. However, permeability and selectivity are both temperature-dependent. It is desired to develop membrane materials with a high selectivity (efficiency) for the desired component, while maintaining a high permeability (productivity) for the desired component.
The relative ability of a membrane to achieve the desired separation is referred to as the separation factor or selectivity for the given mixture. There are however several other obstacles to use of a particular polymer to achieve a particular separation under any sort of large scale or commercial conditions. One such obstacle is permeation rate. One of the components to be separated must have a sufficiently high permeation rate at the preferred conditions or else extraordinarily large membrane surface areas are required to allow separation of large amounts of material. Another problem that can occur is that at conditions where the permeability is sufficient, such as at elevated temperatures or pressures, the selectivity for the desired separation can be lost or reduced. Another problem that often occurs is that over time the permeation rate and/or selectivity is reduced to unacceptable levels.
A further problem that can occur is that one or more components of the mixture can alter the morphology of the polymer membrane over time. This can degrade the permeability and/or selectivity characteristics of the membrane. One specific way this can happen is if one or more components of the mixture causes plasticization of the polymer membrane. Plasticization occurs when one or more of the components of the mixture causes the polymer to swell. Swelling tends to result in a significant increase in permeabilities for most if not all of the components of a feed mixture. This causes degradation of membrane properties. In particular, selectivity is compromised.
It has been found that CO2 can induce plasticization in many polymers, decreasing performance of the membranes made from the polymers. This can be particularly problematic in separations in which CO2 is to be separated from natural or flue gas mixtures. For example, in applications where CO2 is to be separated from flue or natural gases, both high CO2 fluxes through the membrane and high CO2/non-polar gas selectivities are desired when membranes are used that favor CO2 transport through the membrane. However, the selectivity for CO2 can be lost due to plasticization.
Accordingly, there is a strong need for separation membranes with stable, long lasting membrane properties that can separate acid gases such as CO2 from other gases and yet are highly resistant to CO2 induced plasticization. Additionally, there is a strong need for a polymeric membrane that exhibits high selectivity for the separation of gas mixtures of over a wide temperature range, thereby maintaining high selectivity at different process conditions and temperatures common to an industrial gas separation process.